Soft X-ray spectro-ptychography of boron nitride nanobamboos, carbon nanotubes and permalloy nanorods

First B-edge (192 eV) soft X-ray ptychography, challenges related to weak scattering, measurement approaches and reconstructions in energies as low as 180 eV are discussed.

2 Section SI-1 Experimental acquisition and PyNX reconstruction parameters Spot size -diameter of the beam, Overlap factor (%) -(1 -stepsize/spot size) x 100, Overlap factor (k -multiplicative factor) -determined from the plot shown in the main text Fig.7(b), Reconstruction pixel size -Determined from the sample detector distance and pixel size of the camera, AP -Alternate projection, DM -Difference map, Number following the algorithm represents the number of iteration/cycles (http://ftp.esrf.fr/pub/scisoft/PyNX/doc/scripts/index.html). @ The pixel size for stacks varies by +/-0.2 nm depending on the photon energy.

SI-2 Diffraction signal before and after background subtraction
In these measurements which intentionally used low beam intensity, the electronic background is a relatively large fraction of the camera signal. The pattern and magnitude of the background are quite stable. The average of 50 background ('dark') images is recorded regularly. Prior to reconstruction, this background image is subtracted from each of the diffraction images (DI) that constitute a ptychographic image. Fig. S1 is an example. Fig. S1a is a singe DI (on log10 scale) from a ptychographic data set measured when a 1.0 m beam of 192 eV X-rays hits a BN nanobamboo. The patterning in the annulus (the propagation of the zone plate) is a distorted full field signal. Fig. S1b is the background signal (no X-rays) on the same log intensity scale as S1a. Fig S1c is the difference of the DI (S1a) and the background (S1b) (log scale). Figure S1d compares the histograms of Figs. S1a, b,c -see caption for explanation of color coding.  Figure S2(a) shows a STXM image of the CNT#1 sample measured at 350 eV using a 25 nm zone plate. Figure S2(b) and S2(c) show amplitude and phase ptychography images reconstructed from a set of diffraction images (DI) recorded with a 1 µm spot size at 285.2 eV using linear horizontal (LH) polarized light. The dark square in the upper right corner of the ptychography images is carbon build-up from an earlier measurement using a focused spot. The contrast in the phase image is enhanced compared to the amplitude image. One can distinguish between overlapping nanorods due to the strong contrast at the edges of the CNT. Figure S2(d, e) show polarization dependent ptychography images measured at 285.2 eV with LV and LH polarization respectively, and normalized to the optical density units. Figure S2(f) is the XLD map, obtained as the difference of the ptychography images shown in Fig. S2(d) and Fig. S2(e). From the XLD map, one can find the CNT with horizontal orientation have white contrast while the CNT with vertical orientation have dark contrast. The XLD contrast of a CNT is strongest at the C 1s → π* transition at 285.2 eV, which has highest intensity when the X-ray polarization is perpendicular to the long axis of the CNT. There is also considerable XLD contrast at the C 1s → σ* transition at 293 eV, which has highest intensity when the X-ray polarization is parallel to the long axis of the CNT. [S1, S2] For spectro-ptychography we carried out ptychography characterization with photon energies from 280 eV to 304 eV with LV and LH polarization. Then each ptychography amplitude image is reconstructed and normalized to the background. After correcting all the images for the drift with respect to the first image, the pixel intensities highlighted in blue (horizontal tubes) and red (vertical tubes) in Fig. S3(b) are plotted as a function of energy from the image sequence to obtain the spectra shown in Fig.S3(a). The blue line indicate absorption when X-ray polarization is parallel to the orientation of the nanotubes and red line (regions) indicate the absorption when X-ray polarization is perpendicular to the orientation of the nanotubes.  Figure S4 presents absorption and phase images of a region with horizontal and vertical BN nanobamboo over the B 1s →*transition (191.1 to 192.9 eV).

Fig. S4 Absorption and phase imaging of BN nanobamboo across the B 1s →*transition. (a) Absorption and (b)
phase images derived from ptychography data measured using LH polarization, 1 m spot size, 90 % overlap, 100 ms per DI at 9 energies from 191.1 eV to 192.9 in 0.2 eV increments. All images are presented on a common intensity scale, given by the scale bar. Each image is 1050 x 1050 nm. Fig. S5a presents line profile across the edge of a permalloy nanorod from the line indicated in the inset image. The image shown in the inset is the ptychography amplitude image measured at the Fe 2p edge (706 eV) with CL polarization. The y-axis is the amplitude intensity, and the x-axis is position along the length of the line. The abruptness of the decrease in the amplitude across this line (20% -80 %) is 15 nm, which is an estimate of the resolution of this reconstructed image.

Fig. S6
presents the Fourier ring Correlation (FRC) analysis of the spatial resolution for the Fe 2p permalloy sample (Fig. S6a, same image as in Fig. S5) and the B 1s boron nanobamboo (BNB) sample (Fig. S6b, same as Fig. 1e). The crossing of the half-bit curve at 0.12 nm -1 indicates a half-pitch spatial resolution of 8.5 nm for the permalloy sample while the crossing of the half-bit curve at 0.060 nm -1 indicates a half-pitch spatial resolution of 17 nm in the BNB sample.

SI-6 Comparison of diffraction images from permalloy nanorods and BN nanobamboo
Qualitatively the diffraction images (DI) from the polycrystalline permalloy nanorods are richer and extend to higher q than the DI from the BN nanobamboo (BNB) or CNT samples. However, this is difficult to quantify. In addition, the BNB and CNT data sets were recorded at many different energies and two linear polarizations, whereas the permalloy nanorods were only measured at 706 eV, the maximum of X-ray absorption and XMCD signals. The DI intensities and q-range over which statistically significant signal can be measured from the BNB and CNT strongly depend on the DI acquisition time, as well as the photon energy, polarization, and orientation of the sample relative the E-vector, which is why these samples present rich core level absorption spectroscopy, spectromicroscopy and XLD imaging. Figure S7 presents a single DI (on log10 scale) from permalloy (Fig. S7a, S7b) and from BN (Fig. S7c,  S7d). The dynamic range of the signal from the permalloy sample is 4 times larger than the dynamic range of the signal from the BNB sample. However, the q-range over which diffraction signal can be seen is similar (see yellow box in Fig. S7a, Fig. S7b.

SI-7 BN nanobamboo measured at N-edge
This image is used to calculate the resolution of the reconstruction of BNB at the N-edge tabulated in Table 1 (in the  main text).